Places a redemption order for the signed-in member. The member spends their reward item balance, or pays a provider in cash, on a product variant. Eligibility, stock and the balance debit are handled together, so an order either goes through completely or not at all. The caller must be a member of the application.

Send an idempotency_key so a retried request can't place the order twice. Repeating a key with the same request returns the original order with status 200 instead of 201. Reusing a key for a different request fails with 409.

An order can contain only one item for now. The response is the order, in the same shape as Retrieve Order, plus new_balance, the member's current balance in the order's reward item. It is null for a cash order.

Errors come back as detail and error_code. A 400 means the request itself is wrong, for example an unknown variant. A 422 means the order is valid but the current state blocks it, for example out of stock, insufficient balance, or a product that is expired or the member isn't eligible for.